488 ADAM SEDGV/IOK. 



derm and endoderm ? It must be remembered that the first 

 somite is separated from the anterior end of the mesoblastic 

 bands before the latter have reached the level of the middle of 

 the blastopore, so that we are now only concerned with the old 

 embryos of Stage a. After prolonged and careful study of 

 a large number of embryos, I have come to the conclusion that 

 this is a point which it is impossible to settle with certainty by 

 a study of preserved specimens. Still there is a certain amount 

 of evidence, which, on the whole, tends to show that the inter- 

 mediate nuclei at the lips of the blastopore do contribute to 

 the bauds. The evidence depends upon the appearances pre- 

 sented by these nuclei at the lips of the blastopore. These 

 are constantly met with in a state of division, i. e. present- 

 ing the figures which are characteristic of binding nuclei. 

 This is shown clearly in many of my figures, e. g. (Part II, 

 fig. 2Qb; and PI. XXXIV, fig. 3, of this paper). Further, 

 immediately within these intermediate nuclei, and lying be- 

 tween the ectoderm and endoderm, there are often to be 

 een nuclei, which may fairly be supposed to have been 

 erived by division from the intermediate nuclei. These are 

 present in sections through the young mesoblastic bands (Part 

 II, fig. 26 b), and also in sections in front of the latter 

 (PI. XXXIV, fig. 3, close to d. n.) ; in the former case in- 

 dicating that the nuclei in question are reinforcing the nuclei 

 of the mesoblastic bands; in the latter that they are laying 

 down the same structures. 



2. With regard to the second question, I can only state that 

 the completed somites attain their final position by an altera- 

 tion in the relative position of the structures in the anterior 

 region of the embryo. 



PI. XXXIV, figs. 4, 5 a—f, are illustrations of the structure 

 of the embryo at the close of Stage b (Part I, fig. 25). Fig. 4 

 is in front of the mouth, and is through the first pair of somites, 

 now pre- oral. Fig. 5 a is through the mouth and second pair 

 of somites; fig. 5 b between the mouth and anus, through the 

 region in which the blastopore lips have come together and 

 fused ; fig. 5 c is through the anus ; fig. 5 d, e, f, behind the 



